Note: While you can use the computers built-in microphone to record you voice over, you will most likely want to use a USB headset/microphone to reduce room noise while recording.
Plug the USB headset/microphone into the computer. Navagate to the Audio System Preferences for the computer (Apple>System Preferences>Sound). Under the Input tab, select the USB headset/microphone (Each model is labeled differently) and adjust the input volume with the horizontal slider. Speak into the microphone while adjusting the slider and aim to get the meter in the upper 3/4 range. Under the Output tab, Select the USB headset/microphone.
Back in Premiere, go to Premiere Pro>Preferences>Audio Hardware. Set the Default Device pull down menu to your USB headset (Each model of headset is labeled differently )
In the menus go to Sequence>Add tracks. In this window you can create new video and audio tracks. Add "0" video tracks and "1" Audio track. Note the placement option. In this case it will be placed after Audio track 3. Make sure the track type is set to mono (USB headset is a mono microphone).
In the Menus go to Window>Audio Mixer>your current sequence. Here you will see all of the audio tracks in you sequence. In the newly created track click the "R" button to enables recording on that track.
Optional: Clicking the "S" button will mute all other audio tracks (A.K.A. Solo) so you won't hear them while recording and clicking the "M" button will mute the voice over track so you won't hear your own voice while recording which can be distracting.
Optional: You can enable "Meter Input(s) Only" in the small pull down menu located in the upper right corner of the Audio Mixer. This will allow you to see the audio levels from the microphone and make adjustments to your voice or the System Audio Preferences to get the optimal recording level. Aim for 0dB.
Click the red record button at the bottom of the audio mixer window to place Premiere into Record standby mode. Move the playhead in the timeline to where you would like the recording to start. In the Audio Mixer window click the Play button at bottom of the window to start recording. Click the Stop button to stop recording.
The recording will be placed in the timeline and in your Project window.